The present invention relates to a display device using a thin film cathode for emitting electrons into a vacuum from an electron emitting portion and a process for fabricating the display device.
A thin film cathode has, basically, a three-thin-film structure of a top electrode, an insulator (or semiconductor layer or the like), and a base electrode. By applying a voltage between the top and base electrodes, electrons are emitted from the surface of the top electrode into a vacuum. For example, there are an MIM (Metal-Insulator-Metal) type, an MIS (Metal-Insulator-Semiconductor) type, and the like.
The MIM type thin film cathode is described in, for example, Japanese Unexamined Patent Publication No. 7-65710 by the inventors of the present invention. FIG. 2 shows the operating principles of the disclosed technique. When a driving voltage Vd is applied between a top electrode 13 and a base electrode 11 to set the electric field in an insulator 12 to about 1 to 10 MV/cm, electrons around the Fermi level in the base electrode 11 pass through a barrier by the tunnel effect, are injected to a conduction band of the insulator 12 and the top electrode 13, and become hot electrons. The hot electrons are scattered in the insulator 12 and the top electrode 13, thereby losing their energy. A part of the hot electrons having energy of a work function xcfx86 of the top electrode 13 or more is emitted to a vacuum 20.
When a plurality of top electrodes 13 and a plurality of base electrodes 11 are arranged orthogonal to each other in a matrix, a thin film cathode can generate an electron beam from an arbitrary position. Consequently, the thin film cathode can be used as a cathode of a display device or the like. Electron emission has been observed from an MIM (Metal-Insulator-Metal) structure made of Auxe2x80x94Al2O3xe2x80x94Al and the like.
In the case of applying the thin film cathode to a display device or the like, it is desirable to use a thin film cathode having a high electron emission efficiency, that is, a high ratio of emission current to an injection current (diode current). The higher the electron emission efficiency becomes, the more the brightness of the display device improves and, at the same brightness, the power consumption of the display device decreases.
In order to increase the electron emission efficiency of the thin film cathode, it is effective to reduce the thickness of the top electrode 13 as much as possible to reduce energy loss due to the scatter of the hot electrons in the top electrode 13.
For example, Japanese Unexamined Patent Publication No. H2-121227 discloses a technique of forming a thin portion and a thick portion in an electron emitting portion by vacuum evaporation, sputtering, or selective etching using a photoresist.
In the top electrode 13 formed by a conventional thin film forming apparatus, however, when the thickness of the top electrode 13 is reduced too much, the electrode film grows in island shapes on an insulating film, sheet resistance of the electrode sharply increases, and a voltage drop occurs in the surface of the electron emitting portion. Consequently, a problem such that an effective driving voltage Vd cannot be applied to the thin film cathode occurs.
Methods for solving the problem include, for example, a method proposed in Japanese Unexamined Patent Publication No. H2-172127 in which an inclined portion is formed in a thick top electrode so as to expose the surface of a lower insulator and electrons are emitted from a thin portion at the lower inclined portion, and a method proposed in Japanese Unexamined Patent Publication No. H3-55738 in which an opening is formed in a thick top electrode so as to expose a lower insulator and electrons are emitted from the opening.
In the methods, however, it is difficult to assure a metal thin film portion as a key of electron emission with high reproducibility, so that improvement in the electron emission efficiency is limited.
Meanwhile, the inventors of the present invention have proposed a method disclosed in Japanese Patent Application No. 11-191423 in which a pixel is formed by a plurality of thin electron emitting portions to reduce the area of each electron emitting portion, and a thick bus electrode is formed around the electron emitting portion, thereby preventing a voltage drop. The method is preferable since the above-described resistance can be reduced and the metal thin film of the top electrode can be formed thinly independent of the bus electrode. However, when the size of each electron emitting portion and that of the bus electrode are reduced to increase the electron emission efficiency and improve the brightness of the display device, it is feared that required accuracy of alignment increases, and the ratio of the electron emitting portion, that is, the aperture ratio becomes lower. A drastic solving method of further improving the electron emission efficiency and brightness is therefore desired, which is realized by enlarging the area of the electron emitting portion in the thin film cathode as large as possible within the range of the pixel pitch of a display device, for example, to about 50 xcexcm per side which is about the dot pitch of a high precision display device also in a flat panel of a large screen.
The inventors of the present invention also have proposed the technique in Japanese Unexamined Patent Publication No. H8-180794 aiming at high-efficient electron emission, in which fine dots having a height of 20 nm or lower are formed on a top electrode and the external electric field is concentrated onto the dots to thereby decrease the effective work function of the top electrode. It is, however, difficult to obtain a thin film cathode having high electron emission efficiency of 1 to 2% or higher with high reproducibility.
An object of the invention is to improve electron emission efficiency of a thin film cathode and to provide a display device with accordingly improved brightness.
More particularly, an object of the invention is to provide a display device with improved brightness, realized by enabling an effective driving voltage Vd to be applied to a thinned film cathode and irradiating phosphors with electrons emitted through a flat thin film electrode, and a process of fabricating the display device.
Aspects of the present invention disclosed in the specification will be briefly described as follows.
The invention has been achieved by paying attention that the mean free path of hot electrons largely depend on materials used for a thin film electrode in a display device using a thin film cathode for emitting electrons through the thin film electrode into pressure-reduced atmosphere.
Specifically, the electron emission efficiencies of various materials used for a top electrode were analyzed, and it was found for the first time from the analysis that due to a very short mean free path, which is about 0.5 to 5 nm, of a hot electron in the top electrode until the hot electron is scattered, the electron emission efficiency deteriorates. The invention has been made on the basis of an idea that the top electrode has to be made thinner than a conventional one. The object is achieved by positively providing the top electrode with a flat thin film part which is thinner than 5 nm, and irradiating the surface of a phosphor disposed so as to face the top electrode with electrons emitted through the flat thin film part to a vacuum.
A display device according to the invention is constructed in such a manner that a top electrode having a common flat thin film part and a plurality of island parts is disposed apart from a base electrode, a bus electrode for power supply connected to the flat thin film part is provided, phosphors are disposed over the top electrode and the bus electrode and, by applying a voltage between the base electrode and the bus electrode, the phosphors are irradiated with the electrons emitted through the flat thin film part into a vacuum.
With such a configuration, the flat thin film part through which hot electrons are emitted can be positively made thin independently of the bus electrode for power supply. The electron emission efficiency is improved, thereby enabling the display device with improved brightness to be realized.
An area occupied by the surface of the flat thin film part in the top electrode, that is, an effective electron emission area is set to be larger than an area occupied by the island parts at a surface level of the flat thin film part, thereby achieving an electron emission efficiency equivalent to that in the case where the thin film part is provided almost on the entire face of the top electrode.
The bus electrode has a two-layer structure of a thin lower layer connected to the flat thin film part serving as the electron emitting part and a thickly-formed upper layer serving as a low-resistant power supply part, thereby preventing discontinuation of the thin film electrode of the invention at an electric connection part and preventing a voltage drop due to sheet resistance in a large display device.
The island part itself formed in the top electrode on the electron emitting part defined by a protective layer and the like is not structurally in contact with the bus electrode for supplying a potential to the top electrode, and is physically apart from or independent of the bus electrode.
The thickness of each of the plurality of island parts projected from the surface of the common flat thin film part for emitting electrons (that is, the height from the surface level of the flat thin film part) is preferably larger than the thickness of the flat thin film part and also smaller than the thickness of the bus electrode for power supply. The invention is not limited to the arrangement.
According to the invention, the top electrode in the thin film cathode is formed by stacking, for example, a thin film of iridium (Ir) having a thickness of about 1 nm, a thin film of platinum (Pt) having a thickness of about 1 nm, and a thin film of gold (Au) having a thickness of about 2 to 3 nm in this order on an insulating layer, a semiconductor layer, a porous semiconductor layer, or a mixed film or stacked film of the layers provided on the base electrode, and performing a heat treatment. By the heat treatment, cohesion of the Au thin film is progressed around a small part of the Ir thin film as a nucleus and a plurality of island parts made of Au and Ir are formed. Among the island parts, a flat common thin film part from which the Au component is reduced by an amount of the cohesion and which becomes thinner than 5 nm is obtained with good reproducibility in a state where the plurality of island parts and the flat thin film part integrally exist. That is, by the heat treatment, the thin film is reconstructed and further reduction in the film thickness can be realized.
In order to effectively reduce the thickness of the flat thin film part by the formation of the island parts in the heat treatment, it is preferable to preliminarily thinly form each of the Ir film and the Pt film with a thickness of about 1 nm and the Au film with a thickness of about 1 to 3 nm.
From the analysis of the result of the heat treatment, Ir functions as a growth nucleus for cohering Au and forming an alloy, and Pt disturbs the contact between Ir and Au, thereby suppressing formation of an alloy. It is estimated that Pt has an action of suppressing or controlling the cohesion of Au.
The heat treatment is not therefore limited to the combination of Irxe2x80x94Ptxe2x80x94Au. For example, a combination of Ir and Au without using Pt by controlling time and temperature of the heat treatment may be also used. Alternately, at least two kinds of other conductive materials having the above-described functions may be also employed.
According to the invention, a display device using a thin film cathode in which a top electrode is provided over a base electrode, a phosphor is disposed above the top electrode so as to face the top electrode, and pressure-reduced atmosphere is air-tightly kept in a space between the top electrode and the phosphor, is fabricated as follows. The top electrode is formed by providing a thin metal film on the base electrode, and then performing a heat treatment on the thin metal film such that a metal in the thin metal film coheres partially to thereby form island parts in the thin metal film, and make a part thereof remaining as a flat part thinner than initial thickness of the thin metal film. In such a manner, the display device can be easily fabricated with a high yield.